1. Field of the Invention
This invention relates to valves for use in the wall of a string of tubulars, such as casing that is placed in wells. More particularly, a valve is provided that can be opened by a selected pressure inside the tubular and that may remain open.
2. Description of Related Art
To produce hydrocarbons from some reservoirs in the earth, long, horizontal holes are drilled through productive rock. To prevent collapse of the surrounding rock into the horizontal wellbore, casing must be placed in the holes before fluids can be withdrawn. The casing is normally pushed along the horizontal hole, but the frictional resistance between casing and the wall of the wellbore limits the distance that casing can be placed in horizontal wells using standard methods. One way that has been used to allow longer horizontal sections of casing in wells is to float the casing into the horizontal section using a low-density fluid inside the casing. In all wells, horizontal, deviated and vertical, openings in the wall of the casing must be provided for injection of fluids or production of fluids through the well.
After casing is placed in a well, in some wells cement is pumped down the casing and up the annulus between the casing and the wall of the wellbore. Openings for fluid flow through the wall of the casing are commonly made by perforating guns, which shoot a hole through the casing wall, the cement layer and a short distance into the surrounding rock. In other wells, an “open hole” is left outside the casing (no cement) and packers, made of a rubber sleeve that can be inflated or swellable material are placed at selected distances along the wellbore to prevent flow along the annulus outside casing. In this case, valves in the casing wall that can be opened by a mechanical device or by pumping balls down the casing are normally provided between the packers.
European Patent EP 0 681 088 discloses an annulus pressure-responsive valve that can be locked in the open position and then closed. The valve uses a power piston in a housing with an actuating piston. European Patent Application 2 458 139 discloses a valve that can be inserted into the wall of casing in a well by using the valve body to drill through the casing wall. UK Pat. App 2464009 discloses a method of using inflow control devices, which are commonly used to control the rate of flow into casing when there are multiple points of entry. The inflow control devices do not stop flow. U.S. Pat. Nos. 5,957,197 and 6,820,697 disclose downhole valves. It is common to place valves such as sliding sleeves in casing, which can be operated by mechanical tools inside the casing. Such device is disclosed in WO 2012/080487. Valves in casing operated mechanically by running tools or coiled tubing inside casing are often used in connection with hydraulic fracturing of multiple zones in a well. Many patents disclose downhole safety valves, which serve primarily the purpose of effecting a reliable positive closure of the bore of a production tubing string in the event of an emergency.